Presently known two-stage servo valves quite often use a torque motor in the first stage. Typically, the torque motors that are used include a hydraulic amplifier that is configured as a jet pipe or as a flapper between two nozzles. In both cases, the hydraulic amplifier generates a pressure differential between opposite ends of a second stage spool valve to position it to one of two positions. The jet pipe configuration generates this differential by directing a high velocity jet of fluid into one receiver port while venting the other to low pressure. The flapper/nozzle configuration is more complicated since it relies on two fixed area orifices and two variable area orifices. These four orifices are arranged in a bridge type of configuration with high pressure fluid being supplied to each fixed area orifice. The variable area orifice generates high pressure in one chamber by restricting the flow out of one nozzle, and vents the other chamber.
Regardless of the hydraulic amplifier configuration, there is a constant or quiescent flow of fluid even when the second stage spool is stationary. This constant flow needs to be produced by a pump, such as a main engine pump, and thus continually draws horsepower without producing any motion or work. One method to improve engine efficiency is to minimize these sacrificial losses.
Another type of two-stage servo valve is configured with two control ports. One control port is connected to one end of an actuation piston and the other control port is connected to the other end of the piston. With this type of servo valve the high and low pressure sources are simply reversed to each control port to drive the actuator in one direction or the other. However, with this type of servo valve, the first stage still requires a continuous flow of fluid to hold the second stage in position. Some of these valve styles are also designed to latch in the last commanded position so that a continuous application of electrical current is not needed. Although this reduces the need for electric power, it simply holds the first stage against stops so that the pressure levels are maintained at each end of the spool. Moreover, the first stage still requires a continuous flow of fluid. The addition of latching also increases the level of complexity and cost of the torque motor. This is especially the case with the jet pipe configuration which inherently does not have hardened surfaces to hold the armature in place for long durations.
Hence, there is a need for two-position, two-stage servo valve that does not rely on a continuous flow of fluid and/or does not require fixed area orifices and/or does not rely on the cost and complexity of latching. The present invention addresses at least these needs.